Billy Connolly - the Big Yin - is rarely a man to be argued with. I remember a friend's parents who went to see Connolly in concert in Glasgow in the late Eighties. Connolly, dressed in his typically grandstanding manner, mischievously pretended to be hurt at the criticisms in local newspapers about his use of colourful language. 'Does anyone actually have any objections about ma bad language?' he asked his audience. 'Anyone?' My friend's mother, Mrs McLean, bless her, stood up and raised her hand. 'What? You actually object to my language?' asked Connolly, still feigning hurt. 'Well, yes,' she replied. 'Sometimes.' Connolly stared back hard: 'Well you can jest fuck aff, then,' he screamed.

The anecdote leads us to Porno the seventh novel by Trainspotting author Irvine Welsh. Much like the Glaswegian comic Connolly, a 'Weegie' to Welsh, Porno contains all the hallmarks of the now well-established Welsh style. Newcomers and aficionados will be forced to sit down and interpret 'that cunt Donnelly cadge', 'that Chizzie beast', 'ah'm guan', 'it gied ays', 'radges', 'chib' and 'chavvie'.

Welsh, whose first love is music, was always an unlikely ambassador for Scottish literature. Taken in their entirety, Porno and its six predecessors have done as much to popularise the city of Edinburgh as Connolly's humour has Glasgow. Like many of his generation, Welsh, disillusioned by the poverty he saw around him, moved to London to follow the punk movement. He never quite cut it as a musician and instead found himself working as a clerical temp at Hackney Council. Further disillusionment set in and, in the late Eighties, Welsh returned to the Leith area of Edinburgh and began to write short stories while taking a business studies degree.

Edinburgh had changed, however. Now booming with a tourist economy, it was beginning to enjoy all the fruits of a hedonistic, night-time dance music culture. Illegal raves, attended by tens of thousands, were organised throughout the countryside. They attracted all manners of artists, poets, intellectuals, film-makers and authors. And in Edinburgh itself, writer and activist Kevin Williamson had set up the underground literary magazine Rebel Inc.

Rebel Inc, without a doubt, brought Welsh to his first audience - his first short stories for the magazine showed him to be a quirky, often hilarious writer. I was freelancing for a local Glasgow-based magazine, M8, at the time and remember Welsh contributing a few pieces on the importance of dance culture. His journalism was like a rollercoaster of emotions, opinionated and compelling.

But dance music was merely a diversion - a populist front for attracting readers to his writing. Trainspotting, published in 1993, remains his greatest achievement. Set among the working class poor, it chronicled the degraded and apolitical lifestyles of a group of Scottish drug addicts.

It arrived in Scotland just as youth culture and politics were being stirred into one potent mixture. British dance groups were the new punk stars of the day. And Trainspotting, capturing the neurotic and deprived underside of a city suffering from an identity crisis, perfectly documented the isolation of a generation. Unsurprisingly, it is a novel whose legacy Welsh has found difficult to surpass.

Trainspotting would see Welsh assume, perhaps rightly, the role of a British literary maverick. It also, on the evidence of a return to form with Porno, led him towards an anxiety as a writer. His next novel, Marabou Stork Nightmares, was a disappointment. And Ecstasy, a collection of three novellas, was equally clumsy in finding its stride. His fourth novel, Filth, pushed him further away from the limelight. And while his one attempt at literary reinvention, Glue, a sprawling tale of four childhood friends from the housing estates of Edinburgh that spanned nearly four decades, was ambitious, it was released to a near-deafening silence.

With Porno, however, Welsh has gone full circle - both creatively and mentally. The book brings him back to the now familiar, and therefore slightly less shocking, territory of Trainspotting. That book culminated with Renton abandoning his friends, Sick Boy, Spud and Begbie. Ten years later, Porno opens with the various characters from Trainspotting still banished to their own personal hells: Sick Boy is working in a London strip club. Begbie, thankfully, is in a Scottish jail, awaiting release. Renton is hiding in Amsterdam. And Spud, always the hapless victim, is planning to write a history of Leith.

Like Trainspotting before it, Porno opens with a devilishly simple plot. Sick Boy takes over his aunt's dilapidated Leith pub. The site reunites the old cast, while also introducing a slew of fresh new faces. Here, Welsh establishes himself as something of a Renaissance man, expertly, and, for the first time, writing sympathetically about women. He introduces Nikki Fuller-Smith, a sexually voracious film school student who works in a massage parlour to earn money. Smith finds her calling in pornographic movies. She is the most well-rounded character yet to surface in a Welsh novel:

'The most horrible thing a man can say to me is that I've got a great body. Because I don't want a good, great, lovely, beautiful, body. I want a body good enough to be in the magazines and if I had one I would be in them and I'm not cause I don't. My mascara's running with my tears, and why am I crying? Cause I'm going nowhere, that's why.'

The passage, and Smith's fading dreams, sums up Porno. Back for the first time since 1993 on friendly ground, Porno, like Trainspotting, captures and celebrates the hangover of youth.